When Docker released its latest version, Docker Engine v1.12, it included quite a few changes to the capabilities provided by Docker Swarm. In today’s article, we’ll be exploring how to deploy a service using Docker’s Swarm Mode.

Activating Swarm Mode on Ubuntu 16.04

Before we can deploy a service on a Docker Engine Swarm, we will first need to set up a Swarm Cluster. Since we’re showing capabilities added with 1.12, we will also be installing the latest version of Docker Engine.

The following instructions will guide you through installing Docker Engine on Ubuntu 16.04. For other versions and platforms, you can follow Docker’s official installation guide.

Setting Up the Docker Apt Repository

For this installation, we’ll be using the standard installation method for Ubuntu, which relies on the Apt package manager. Because we will be installing the latest version of Docker Engine, we need to configure Apt to pull the docker-engine package from Docker’s official Apt repository rather than the systems preconfigured repositories.

Add Docker’s Public Key

The first step in configuring Apt to use a new repository is to add that repository’s public key into Apt’s cache with the apt-key command.

The above apt-key command requests the specified key (58118E89F3A912897C070ADBF76221572C52609D), from the p80.pool.sks-keyservers.net key server. This public key will be used to validate all packages downloaded from the new repository.

Specifying Docker’s Repository Location

With Docker’s public key now imported, we can configure Apt to use the Docker Project’s repository server. We will do this by adding an entry within the /etc/apt/sources.list.d/ directory.

When we refresh Apt’s package cache, Apt will look through all files within the sources.list.d/ directory to find new package repositories. The command I just showed you creates (if it doesn’t already exist) a new file called docker.list with an entry that adds the apt.dockerproject.org repository.

Updating Apt’s package cache

To refresh Apt’s package cache, we can run the apt-get command with the update option.

apt-get update

This will cause Apt to repopulate its list of repositories by rereading all of its configuration files, including the one we just added. It will also query those repositories to cache a list of available packages.

Installing the linux-image-extra prerequisite

Before installing Docker Engine, we need to install a prerequisite package. The linux-image-extra package is a kernel specific package that’s needed for Ubuntu systems to support the aufs storage driver. This driver is used by Docker for mounting volumes within containers.

To install this package, we will use the apt-get command again, but this time with the install option.

apt-get install linux-image-extra-$(uname -r)

In that apt-get command, the $(uname -r) portion or the command will return the kernel version for the current running kernel. Any kernel updates to this system should include the installation of the appropriate linux-image-extra package version that coincides with the new kernel version. If this package is not updated as well, some issues may arise with Docker’s ability to mount volumes.

Install Docker Engine

With Apt configured and the linux-image-extra prerequisite package installed, we can now move to installing Docker Engine. To do so, we will once again use the apt-get command with the install option to install the docker-engine package.

apt-get install docker-engine

At this point, we should have Docker Engine v1.12.0 or newer installed. To verify that we have the correct version, we can execute the docker command with the version option.

With this, we can see that both the Server and Client versions are 1.12.0. From here, we can move on to creating our Swarm.

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Creating a Docker Swarm

From this point on within this article, we will be executing tasks from several machines. To help make things a bit more clear, I have included the hostname in the command examples.

We will start our Swarm Cluster with two nodes. At this point, both of these nodes should have Docker Engine installed based on the instructions above.

When creating the Swarm Cluster, we will need to designate a node manager. For this example, we’ll be using a host by the name of swarm-01 as a node manager. To make swarm-01 a node manager, we need to create our Swarm Cluster by executing a command on swarm-01 first. The command we will be executing is the docker command with the swarm init options.

root@swarm-01:~# docker swarm init --advertise-addr 10.0.0.1
Swarm initialized: current node (awwiap1z5vtxponawdqndl0e7) is now a manager.
To add a worker to this swarm, run the following command:
docker swarm join \
--token SWMTKN-1-51pzs5ax8dmp3h0ic72m9wq9vtagevp1ncrgik115qwo058ie6-3fokbd3onl2i8r7dowtlwh7kb \
10.0.0.1:2377
To add a manager to this swarm, run the following command:
docker swarm join \
--token SWMTKN-1-51pzs5ax8dmp3h0ic72m9wq9vtagevp1ncrgik115qwo058ie6-bwex7fd4u5aov4naa5trcxs34 \
10.0.0.1:2377

With the above command, in addition to the swarm init, we also specified the --advertise-addr flag with a value of 10.0.0.1. This is the IP that the Swarm node manager will use to advertise the Swarm Cluster Service. While this address can be a private IP, it’s important to note that in order for nodes to join this swarm, those nodes will need to be able to connect to the node manager over this IP on port 2377.

After running the docker swarm init command, we can see that the swarm-01 was given a node name (awwiap1z5vtxponawdqndl0e7) and made the manager of this swarm. The output also supplies two commands: one command is to add a node worker to the swarm and the other is to add another node manager to the swarm.

Docker Swarm Mode can support multiple node managers. It will, however, elect one of them to be the primary node manager which will be responsible for orchestration within the Swarm.

Adding a Node Worker to the Swarm Cluster

With the Swarm Cluster created, we can now add a new node worker using the docker command provided by the output of the Swarm creation.

In this example, we added swarm-02 to the swarm as a node worker. A node worker is a member of the Swarm Cluster whose role is to run tasks; in this case, tasks are containers. The node manager, on the other hand, has a role of managing the orchestration of tasks (containers) and maintaining the Swarm Cluster itself.

In addition to its node manager duties, a node manager is also a node worker, which means it will also run tasks for our Swarm Cluster.

Viewing the current Swarm nodes

With the previous commands executed, we now have a basic two-node Swarm Cluster. We can verify the status of this cluster by executing the docker command with the node ls options.

From the output of this command, we can see that both swarm-01 and swarm-02 are in a Ready and Active state. With this, we can now move on to deploying services to this Swarm Cluster. We'll cover that next time in part two of this tutorial.